There are a number of systems where information is received by appropriate sensors over a number of channels. Examples of such systems include radar, sonar, and phased array ultrasonic scanners used primarily for medical applications. While the teachings of this invention may find use with any system where information is being received over multiple channels, for purposes of illustration, the following discussion will be primarily with respect to a phased array ultrasonic scanner.
Such scanners may operate with a uniform fixed gain for all receive channels in the array. However, a receiver gain profile that smoothly decreases toward either end of the receiver array will achieve much improved side-lobe performance, although with some widening of the main lobe. This smooth tapering of gain is referred to as apodization, and the shape or characteristic of the tapering will be referred to as the apodization function or profile. There are a number of commonly used apodization functions which are well known from digital signal processing, these functions including the "Hamming function", the "Hanning function", the "Bartlett function" and the "Blackman function". Each of these gives a somewhat different tradeoff between main-lobe width and side lobe level. For purposes of illustration, the following discussion will be with respect to the Hamming function.
It may be desirable in some applications that the receive aperture, or in other words the number of available channels which are being utilized, be held constant. However, for applications such as ultrasonic scanning, where the depth of the scan increases uniformly with time, it is often desirable to maintain a constant f number (distance to the focal point/aperture size) rather than a constant aperture size. For example, for an assumed f number of f2, the aperture size would be maintained at one-half the distance to the focal point. However, since for a phased array ultrasonic scanner, the depth to the focal point increases linearly, constant f operation requires that the size of the receive aperture also be expanded linearly with time. Thus, a constant f receiver might start with only the center element or channel being used at depth 0, with the number of channels used increasing linearly until the depth reaches two times the array size (for an f number of f2) At this time, operation would still be at f2. For deeper depths, the system would return to constant aperture operation. More generally, it might be desirable to have the flexibility to start a scan with a selected aperture, to start constant f operations at a selected point in the scan, and to terminate constant f operations and return to constant aperture at a second, later point in the operation.
Dynamic aperture receiving is made more complicated by the fact that it may be desired to maintain the apodization function intact as the aperture expands. In other words, at every instant in time, the aperture gain on each channel should provide the desired apodization function stretched or compressed to fit the required aperture size at that instant. The aperture gain for channels outside the desired aperture size or window at a given instant should be, as nearly as possible, zero.
To achieve the above objective, each receiver channel needs to be controlled in gain as a function of time. Further, the time history of the gain or gain profile is different for each channel (except, due to symmetry about the center, channels equidistant from the center have the same gain).
Thus, to achieve dynamic apodized receive apertures, a controllable gain amplifier must be provided for each channel, with a means being provided for generating a different, time dependent, control signal for each of the controlled gain amplifiers. The gain desired for each channel is a function of two variables, the aperture position (x) of the channel and time (t) (which is directly related to the depth of scan). The exact function depends on the apodization function utilized. By holding x constant and varying only t for each element or channel in turn, it is possible to obtain the N separate gain control functions of time required to control the N different channels of the system. If the controlled gain amplifiers do not have a linear characteristic, the time functions can be predistorted to compensate for this nonlinearity.
While a computer with, for example, a table look-up ROM or RAM could be utilized to generate the required N time functions, or other similar digital techniques could be utilized to perform this function, such an implementation can be relative large, complex and expensive. It may also be relatively slow in generating the large number of gain control values needed, for example, for a 128 channel system at each given instant, where scanning is being performed rapidly.
Similar considerations may also apply for other values which vary with focal point or depth of scan in a given system, such as frequency, phase or the reduced system gain which arises from the smaller number of channels in a reduced size aperture.
A need, therefore, exists for a relatively simple, compact, inexpensive way to generate dynamic control signals in a multichannel signal receiving system, and in particular, to generate the dynamic gain control signals required to control the gain controlled amplifiers for each channel in such a multichannel system.